1. Field of the Invention
The present invention relates to a zero-voltage switching type electronic ballast for a fluorescent lamp, and more particularly, to a zero-voltage switching type electronic ballast for a fluorescent lamp, wherein a switching operation is performed at the moment when the terminal voltage is zero, and which provides an improvement to the switching control signal generating circuit.
2. Background of Related Art
For the ballasts for use in fluorescent lamps, large and heavy coil ballasts have been replaced with electronic ballasts which are relatively compact and light. A conventional electronic ballast is mainly constituted of the self-oscillation type electronic ballast as shown in FIG. 1. In the self-oscillation ballast of FIG. 1, a direct current (DC) voltage, which is obtained by full-wave-rectifying an alternating current (AC) voltage and smoothing the thus-obtained voltage waveform, is used to charge a capacitor C2 via a resistor R1. When this charging voltage reaches the conduction voltage of a diac DAC, the diac instantaneously applies a DC bias voltage for a transistor Q2, which therefore turns on and begins the self-oscillation. Here, transistor Q2 has an opposite applied base-emitter voltage than a transistor Q1, so that an alternating switching operation is performed.
However, such a self-oscillation type electronic ballast has several drawbacks, including the following.
1) Energy conservation is not achieved even for the variation in low power voltage.
2) Since the switching devices are very inefficient due to the high-frequency operation (considered the most fundamental function of an electronic stabilizer), energy savings is very poor and the reliability of the switching devices is also poor due to a heating of the devices due to the high frequency.
3) The high frequency components (harmonics) produced by the switching operation may cause electrical interference and may produce noise sources (see "High Frequency Resonant Inverter for Group Dimming Control of Fluorescent Lamp Lighting Systems" by K. H. Lee, E. C. Nho and G. H. Cho, IEEE TES Annual Meeting Conference Rec., p149-p154, 1989).
4) Since a high voltage is applied to the filament of the fluorescent lamp in the initial step of discharging, sputtering and subsequent blackening as a result thereof may reduce lamp life.
Accordingly, the inventor of the present invention has suggested a technology (disclosed in Korean Patent Publication No. 91-5118) wherein the driving current is detected prior to discharge and then, if the detected current is larger than a predetermined value, the turn-on time of the switching device is designed to be delayed so that part of the energy stored in the resonant circuit is regenerated as the power source, and therefore the surge voltage induced across the discharge lamp is suppressed to improve the life time of the electric discharge lamp. The inventor has also suggested a zero-voltage switching type current controlled inverter circuit (disclosed in Korean Patent Publication No. 92-4995) wherein a zero-voltage switching method is adopted to lower the slope of the voltage across the switching device, so that harmonics may be removed.
In the Korean Patent Publication No. 92-4995, however, the amplitude and phase of the load current is detected by means of a current transformer, and the thus-detected signal is positively fed back so that the switching devices are controlled by means of alternate zero-voltage switching. Therefore, since the switching operation is performed based on the resonant frequency of the oscillation, the frequency characteristic may vary according to peripheral conditions and component characteristics, which is undesirable. Also, the driving transformer for such a device has a winding structure which is complex because a current transformer, a winding for operation in the pseudo-saturation domain, and a winding for detecting the load current are all necessary. Furthermore, overheating of the switching device and voltage breakdown due to excessive current cannot be prevented in such a conventional device.